The present invention relates to a porous (or microporous) film of a high molecular weight polyolefin suitable for use in filtering materials, separators for aqueous electrolyte batteries and the like and to a process for producing same. More particularly, the present invention relates to a porous film of excellent air-permeability obtained by preparing an air-impermeable sheet or film of a polyolefin from a high molecular weight polyolefin as a starting material without addition of a plasticizer or a solvent and thereafter subjecting the air-impermeable sheet or film to a thermal treatment as well as a process for producing same. The present invention further relates to a porous film of the polyolefin possessing excellent strength and air-permeability obtained by stretching the film in at least one direction after the thermal treatment and to a process for producing same.
A number of processes were already proposed for the production of a microporous film of a high molecular weight polyolefin, for example, as seen in Japanese Patent Publn. No. Hei. 6-53826, Japanese Patent Publn. No. Hei. 6-2841 and Japanese Patent Publn. No. Hei. 7-17782.
In order to obtain a microporous film, all of these processes comprise adding a solvent of hydrocarbon series such as decane, dodecane, decaline, a paraffin oil or a mineral oil; a fatty acid hydrocarbon derivative such as a fatty acid, a fatty acid ester and an aliphatic alcohol; a wax of paraffin series; or a plasticizer composed of a low molecular weight compound such as dioctyl phthalate or dibutyl sebacate to a high molecular weight polyolefin, shaping a film from the polyolefin mixture, and then eliminating the low molecular compound from the film.
Above all, the processes proposed in Japanese Patent Publns. Nos. Hei. 6-53826 and Hei. 6-2841 are characterized by eliminating the low molecular compound from the film while stretching it for obtaining a microporous film of high strength.
As a result of extensive research for obtaining a porous film from a high molecular weight polyolefin without addition of a low molecular weight compound which has to be eliminated after all, it has been found by the present inventors that a porous film of excellent air-permeability can be obtained by subjecting a film of a specific high molecular weight polyolefin film to a thermal treatment for making it porous. It has also been found that a porous film of extremely excellent strength and air-permeability can be obtained by stretching the above porous film in at least one direction. The present invention has been accomplished on the basis of the above finding.